1. Field of the Invention
The present invention relates to a small vibration actuator used in, for example, a vibration generating source for notifying a user of an incoming call of a portable wireless device such as a cellular phone, or a vibration generating source for providing tactile sensation of a touch panel of a portable wireless device or reality of a game played by a game device to a user's finger or hand.
2. Description of the Related Art
Hitherto, as a technology in such a field, the technology described in Japanese Unexamined Patent Application Publication No. 2003-220363 (PTL 1) is provided. The vibration actuator described in PTL 1 includes a cup-shaped yoke in a housing (frame). A circular cylindrical magnet is secured to one side of a bottom portion of the yoke and a weight is secured to the other side of the bottom portion of the yoke. A coil is disposed between the magnet and the yoke. In a direction of a vibration axis, a first compression coil spring is disposed between an end portion of the housing and the weight, and a second compression coil spring is disposed between the housing and an open-side end portion of the yoke. By interposing the weight and the yoke between the first compression coil spring and the second compression coil spring in this way, the vibration actuator is reduced in size. In order to prevent rattling of the first compression coil spring, the weight is provided with a protrusion that is fitted into an end portion of the first compression coil spring.
However, in the aforementioned existing vibration actuator, since, in order to prevent rattling of the first compression coil spring, the protrusion of the weight is fitted into the first compression coil spring, an outer peripheral surface of the protrusion of the weight continues contacting an end portion of an inner peripheral surface of the first compression coil spring that is repeatedly stretched and contracted when the weight vibrates. As a result, the first compression coil spring that is repeatedly stretched and contracted when the weight vibrates rubs against or collides with the outer peripheral surface of the protrusion. This may generate abnormal noise from the first compression coil spring at a portion where the first compression coil spring contacts the outer peripheral surface of the protrusion. Further, when the first compression coil spring rubs against or collides with the protrusion, the amplitude of vibration of the weight is affected, thereby causing the vibration quantity to become changeable. In particular, such a problem tends to occur in a small vibration actuator because the wire diameter of the compression coil spring is very small and the compression coil spring, itself, is easily twisted.
With regard to the weight disclosed in PTL 1, there is no technical idea of effectively using the protrusion of the weight as a portion of the weight, and a spring seat is merely formed by forming an end portion of the weight into a concave shape. The evidence for this is that even an end portion of the second compression coil spring that is disposed between the housing and the open-side end portion of the yoke is held by a spring seat having the same shape as the spring seat of the first compression coil spring.